Drug interactions between intraoperative epidural fentanyl and meperidine (pethidine) added to epidural local anesthetic agent and postoperative epidural morphine for potentiation of the quality of intraoperative and postoperative analgesia during Cesarean section are controversial. The purpose of this study was to determine the influence of epidural fentanyl or meperidine on the efficacy of epidural local anesthetics and morphine during and after Cesarean section, respectively. Epidural anesthesia was established using a local anesthetic mixture of 2% lidocaine 10 ml and 0.5% bupivacaine 20 ml with epinephrine 1: 200,000 by fractionated dose administered via epidural catheter to anesthetize the patient up to T-4 sensory dermatome in 107 patients scheduled for elective Cesarean section. After delivery, 107 patients were received one of the following study drugs epidurally in a randomized, double-blind manner: saline 10 ml in saline group (n=50), fentanyl 100 ug/10 ml in fentanyl group (n=33), and meperidine 25 mg/10 ml in meperidine group (n=24). All patients were received 3.5 mg of morphine diluted with saline 6 ml epidurally just after peritoneal closure. Additional administration of the epidural fentanyl or meperidine didn't influence analgesic effect of the local anesthetics injected epidurally. In fentanyl group, postoperative pain, nausea, and sedation were more severe than saline group. In meperidine group, the incidences of intraoperative nausea and vomiting were highest among the groups, but the degree of postoperative pain, nausea, itching, and sedation were not significant. In conclusion, intraoperative injection of epidural fentanyl or meperidine were ineffective for the efficacy of epidural local anesthetics and morphine.